One by one, three bee-eater chicks hatch in their underground nest, and they are all about to die. There is a fourth chick in with them, but it is no bee-eater. It is a greater honeyguide, a different species that was laid in the nest and mistakenly incubated by the bee-eater parents. Having hatched a few days ago, it has been lying in wait for its foster siblings. As each emerges in turn, the honeyguide attacks it with vicious spikes on its bill. Completely blind, it is literally stabbing in the dark but it makes up for its imprecision with brutality. Within minutes of entering the world, the other chicks are dead.

Warning: this video is not pleasant. Watch at your own risk.

Greater honeyguide are known for leading honey badgers and humans to bee hives. In return, they eat some of the honey liberated by their more powerful partners. That’s the adults – the chicks are more mysterious. People have only seen youngsters from five of the 17 honeyguide species and all of them have been found in nests of other birds with no native chicks around. And all of them have savage bill hooks.

The natural assumption was that they are ‘brood parasites’ – birds that lay their eggs in the nests of others. Like the more famous cuckoos, they foist their eggs onto other birds, turning them into unwitting surrogate parents. And like the cuckoos, the honeyguide chicks monopolise their foster parents’ attentions by killing their siblings. A common cuckoo chick will push the eggs and young of its host out of the nest. The honeyguides, with their bill hooks, achieve the same end by stabbing rather than defenestration.

Until now, this behaviour has only ever been described once, sixty years ago. A man called Gordon Ranger watched a honeyguide chick bite another to death in the palm of his hand and, somehow, the chick managed to stab its bill hook through Ranger’s tongue. Now, Claire Spottiswoode from the University of Cambridge and Jeroen Koorevaar from E.C.O. Logics have managed to film the killer chicks in action.

They snuck infrared cameras into the underground nests of bee-eaters, built within aardvark burrows. “It was incredibly exciting when Jeroen and I watched the honeyguides’ behaviour for the first time,” says Spottiswoode. Their footage revealed scenes of breaking and entering, property damage, and homicidal violence.

The greater honeyguide are clearly a massive problem for bee-eaters. They visit almost two thirds of bee-eater nests and when they lay their own eggs, they puncture those of their hosts. Many, but not all, are destroyed. In response, more than half of the bee-eaters outrightly abandoned their nests but 29 percent stayed behind and incubated the alien eggs alongside their surviving clutch.

The honeyguide egg always hatched first, by two to four days. Their bill hooks were well developed by the time the bee-eater chicks emerged, and they weighed three to six times more. The honeyguides quickly mauled their foster siblings, shaking brutally and striking haphazardly for a few minutes. The defenceless bee-eaters could do nothing.

“They’re pretty brutal to watch, and all the more so for the fact that the chicks are newly hatched and in darkness,” says Spottiswoode. But she is philosophical about it. “These species have been coevolving with one another for millions of years. This is part of their lives, and their adaptations are fascinating to me.”

The honeyguide chicks will even sink their hooks into human hands. Their bites, while seldom deep enough to draw blood, are strong and “moderately nasty”. Only the foster bee-eater parents get an amnesty from the snapping bill-hooks.

Once their foster siblings are dead, the honeyguides have no need for their bill hooks and like many murderers, they ditch their weapons. As the chicks grow, the hooks eventually become absorbed into the growing bill. One month later, the adults carry no trace of the weapons used in their earliest days.

It might seem that there are no drawbacks to the honeyguide’s strategy – with seemingly little effort, it monopolises its new parents. But Spottiswoode thinks that there is a cost. Nests of chicks furiously beg for food when the parent birds return to the nest. These cries are irresistible triggers that compel the parents to feed their brood. By killing off the competition, the honeyguides also kill off companions that could help them to beg more effectively.

They might have a way around that. Spottiswoode says that the honeyguide chicks make a distinctive rapid begging call. “To our ears at least, it sounds remarkably like a whole brood of very hungry bee-eaters, and might stimulate the parents to bring food at correspondingly high rates.” That’s speculative, but Spottiswoode will no doubt test it in future studies. She is also working on how the bee-eaters have counter-adapted to their murderous parasites.

Update: Corrected opening paragraph to reflect the fact that the honeyguide chicks hatch one at a time rather than synchronously, and a later para to show that honeyguides don’t actually guide honey badgers.